Neutron scattering by low-energy excitations of disordered materials

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Abstract
An account of inelastic neutron scattering is presented, with emphasis on the derivation of the vibrational density of states for disordered systems. A study of low-frequency excitations in an amorphous material which scatters coherently reveals the character of these vibrational modes through a careful analysis of the inelastic structure factor of the various modes. Results obtained on vitreous silica are analysed in conjunction with heat capacity measurements to provide a microscopic description of these modes. Inelastic scattering from hydrogenous materials gives directly the density of states. Such measurements made on epoxy resins and on the amorphous and crystalline monomer basis of these resins can be interpreted in terms of phonon-fracton cross-over. An experiment on a fractal system made of silica smoke-particle aggregates shows that further investigations are needed to understand the nature of the vibrational modes in these networks and the role of anharmonicity.